Laser for use in non-linear optics

ABSTRACT

The combination of coupling, modes and a Q-switch allows amplitude modulated laser pulses to be produced which contain a high output capability and high repetitive rate. The envelope of the pulse of the coupling modes in the shape of a Q-switch-pulse varies in intensity. Preferably, said combination is achieved by using, a satiated semiconductor absorber (SESAM), whereby the parameters thereof must be specially adjusted to the other elements used, so that the requirement for high repetitive Q-switch-pulse rates can be met.

[0001] The invention relates to a laser having a high output power andhigh repetition rate for use in non-linear optics as claimed in thepreamble of claim 1, a method for producing laser pulses as claimed inthe preamble of claim 15, a use of a laser for producing UV emissions asclaimed in claim 18 and a frequency conversion light source as claimedin claim 19.

[0002] Solid-state lasers having high peak outputs are used in manyapplications. The high output power is particularly suitable forutilizing non-linear optical effects. For example, it is possible toproduce UV light by guiding the emission of an Nd:YAG or Nd:vanadatelaser with a Q-switch through two non-linear optical crystals with anarrow focus within these crystals. The third harmonic is generatedthereby which leads to a strong emission with frequency tripling and awavelength of 355 nm. In a similar manner, it is also possible toproduce 266 nm or even shorter wavelengths. Such Q-switch lasers can bepumped by means of flash lamps or laser diodes.

[0003] However, these lasers have the disadvantage that the repetitionrate of the pulses is limited to a range of up to a few tens ofkilohertz, unless a substantial reduction in the peak power is accepted.In order to achieve higher repetition rates at high output powers,Nd:YAG- or Nd:vanadate-based lasers are operated by means ofmode-locking. However, these lasers usually have substantially longerresonators of the order of magnitude of 1.5 m or longer and aretherefore not suitable for applications requiring particularcompactness. On the other hand, achievable peak output power decreasesif resonators for continuous-wave (cw) mode-locking are designed to beshorter.

[0004] The simultaneous and accidental occurrence of mode-locking andQ-switch is known from the prior art and is felt to be problematic sincethe special mode-locking or Q-switch properties desired in each case areadversely affected. Consequently, efforts are being made to avoid theoccurrence of a mixed mode comprising mode-locking and Q-switch (cf. C.Hönniger et al., “Q-switching stability limits of continuous-wavepassive mode-locking”, J. Opt. Soc. Am. B/Vol. 16, No. 1/January 1999).

[0005] U.S. Pat. No. 6,252,892 discloses, for example, a resonantFabry-Perot saturable absorber (R-FPSA) which affects mode-locking in alaser. A two-photon absorber (TPA) is especially used in order toprevent a parallel occurrence of Q-switching.

[0006] On the one hand, particularly for applications in the area ofnon-linear optics, mode-locked or Q-switched lasers of the prior art aresubject to a compromise between high pulse rate and high pulse power,which is additionally subject to design restrictions. On the other hand,in the case of lasers of the prior art, attempts are made absolutely toavoid a resultant combination of mode-locking and Q-switching.

[0007] An object of the invention is to provide a laser sourcesimultaneously having high pulse repetition rates greater than 100 kHzand high output power or peak power, which laser source is suitable inparticular for utilizing non-linear optical effects, such as, forexample, UV generation or two-photon or multiphoton absorption.

[0008] A further object of the invention is to provide a laser sourcewhich is at least comparable in terms of its power with mode-lockedlasers to date but nevertheless permits a more compact design.

[0009] These objects are achieved, according to the invention, by thedefining features of claims 1 to 15, respectively. Advantageous andalternative embodiments and further developments of the apparatus and ofthe method are evident from the features of the subclaims.

[0010] The invention is based on the concept of achieving, by acombination of Q-switch and mode-locking, a compact laser design whichcombines high pulse repetition rates of more than 100 kHz withsimultaneous, high output powers. The laser emission can particularlyadvantageously be used for utilizing non-linear optical effects.

[0011] One embodiment comprises a diode-pumped solid-state laser havinga particularly compact resonator (length <0.75 m), which includes asaturable absorber and which is operated in a mode which represents acombination of Q-switch and mode-locking, the time between themode-locked pulses being in the nanosecond range and corresponding tothe resonator revolution time, while the repetition rate of the Q-switchpulses is of the order of magnitude of a few hundred kilohertz and leadsto a modulation of mode-locked pulses in which the Q-switch pulsesrepresent the envelope of the mode-locked pulses. This combination ofQ-switch and mode-locking leads to a sequence of ultra-short pulses.Since the emittable power approximately corresponds to the integral overthe pulse duration, there is a relationship between pulse height andpulse duration. With this design, it is therefore possible to achievepeak powers in the region of a few kW. With this power and the possible,compact design, the laser is suitable in particular for applications ofnon-linear optics and for applications with particular requirements forthe size of the laser used.

[0012] In mode-locked lasers, the pulse repetition rate is determined bythe resonator revolution time, which in turn depends on the cavitylength. On the other hand, the repetition rate of the pulses of Q-switchlasers is determined via the pulse build-up, especially by the followingpoints:

[0013] A pulse build-up begins only when the inversion is pumped up tosuch an extent that the amplification associated therewith compensatesthe -revolution losses. In the preferred embodiment, the revolutionlosses comprise the sum of the proportions of the losses due to couplingout, modulation depth of the saturable semiconductor absorber andfurther losses. This means that lower revolution losses lead to a higherrepetition rate of the Q-switch pulses.

[0014] This is offset by the power pumped into the laser medium. Withincreasing power, i.e. pump intensity, the repetition rate of the pulsealso increases.

[0015] The occurrence of noise-free operation of a laser withmode-locking and Q-switch cannot always be expected but can be achievedif, as a rough estimate, at least one photon from a preceding pulseremains in the resonator and initiates the pulse build-up.

[0016] From the mathematical relationship between the individualquantities, it follows as a condition that${\frac{E_{Photon}}{h \cdot v} \cdot \left( {1 - {loss}} \right)^{\frac{T_{qs}}{T_{res}}}} > 1$

[0017] where the fraction on the left indicates the number of photons,“loss” denotes the revolution losses and T_(qs) and T_(res) denote theperiod of the Q-switch pulse and the resonator revolution time,respectively. In order to obtain the intended noise-free repetition rateof the Q-switch envelopes of mode-locking, this condition must befulfilled.

[0018] The laser according to the invention is described in detail belowpurely by way of example with reference to embodiments shownschematically in the drawing. Specifically,

[0019]FIG. 1 shows the schematic diagram of the design of a laseraccording to the invention, with a subsequent use for frequencymultiplication of the emitted light in the UV range;

[0020]FIG. 2 shows the schematic diagram of an example of a preferredembodiment of the laser according to the invention;

[0021]FIG. 3 shows the diagram for the measurement of an MLQSW pulseseries of a laser according to the invention and

[0022]FIG. 4 shows the schematic diagram of a possible application of alaser according to the invention for curing UV-sensitive synthetic resinor for optical data storage in three dimensions or the like.

[0023]FIG. 1 shows the schematic design of a mode-locked Q-switch(MLQSW) laser and the possible design of an arrangement for utilizingnon-linear optical effects with the use of the MLQSW laser according tothe invention, e.g. for frequency multiplication in a frequencyconversion light source according to the invention.

[0024] The laser substantially comprises a pump laser source 1, e.g. alaser diode, which pumps, via a transmission optical system 2 and adichroic mirror 3, a laser medium 5 which is part of the laserresonator, it also being possible for the laser medium 5 to representone end of the resonator. The resonator has the laser mirrors 4 and 7and optionally further beam- or mode-forming optical elements 6 and atleast one saturable absorber element, e.g. a saturable semiconductor(SESAM) which produces mode-locked Q-switch operation of the laser. Inthis example, the saturable semiconductor is part of the mirror 7, whilethe mirror 4 serves for coupling out from the resonator. The light ofthe pump laser source 1 is focused into the laser medium 5 by atransmission optical system 2. The volume excited in the laser medium 5is substantially positioned within the laser mode which is defined bythe mirrors 4 and 7 of the laser resonator. The mirror 4 is partlyreflecting in the region of the laser wavelength and thus serves as acoupling-out device. The mode-locked Q-switch laser emission S ispassed, in the further course after detection by the mirror 8, through asetup comprising a non-linear optical system 9 for frequency conversionof the laser emission, it once again being possible to use furtherbeam-forming optical elements 6. As an alternative, it is also possibleto use the laser emission for focusing into a medium in which two-photonor multiphoton effects locally change the properties of this medium.

[0025] Subsequent frequency conversion of a laser beam from a compactMLQSW laser: The efficiency of a frequency conversion is determined bythe peak power of the incident laser beam. An MLQSW laser therefore hasthe advantage of achieving a higher efficiency than a comparablemode-locked (i.e. with the same cavity length) or Q-switch laser.

[0026] Generation of the second harmonic: This advantage is utilized,for example, in the case of the generation of the second harmonic. Forexample, a mode-locked Q-switch laser which is operated at a wavelengthin the range of 750 nm . . . 980 nm (that is, for example, with adiode-pumped MLQSW CR:LiSAF, Cr:LiCAF, Cr:LiSGAF, Nd:YAG, Nd:vanadate oralexandrite laser which emits with the corresponding line) can produce afrequency conversion in the blue range by a single passage through anon-linear optical medium. Such media are, for example, lithiumtriborate, potassium niobate, etc.

[0027] Generation of the third harmonic: In comparison with thegeneration of the second harmonic, the conversion efficiency of thegeneration of the third harmonic is even more dependent on the peakpower. An efficient UV laser can therefore be obtained by using amode-locked Q-switch Nd:vanadate laser which is frequency-converted in asequence of two lithium triborate crystals. The first crystal generates532 nm laser light from the incident 1064 nm laser light, and the secondcrystal mixes the 532 nm and the 1064 nm light to effect an efficientfrequency conversion into a wavelength of 355 nm. The UV beam (355 nm)can then be used, for example, in an embodiment and can carry in themiddle, for scanning, a focused laser beam through a synthetic resinwhich cures on exposure to 355 nm light. In this way, three-dimensionalstructures can be produced in the synthetic resin. This laser istherefore suitable for stereolithographic applications in which athree-dimensional pattern of cured synthetic resin is produced insideliquid synthetic resin. Owing to the high achievable repetition rateof >100 kHz, there are only a few restrictions compared with theQ-switch lasers with their repetition rates of a few tens of kHz, withrespect to the speeds at which the beam is passed through the syntheticresin. In order to control the structure writing process in the medium,an optical switch can be used in the laser beam part.

[0028] In an alternative form of implementation, a tunable diode-pumpedMLQSW laser, such as, for example, with Cr:LiSAF, can be used in orderto obtain tunable UV light with a high degree of compactness andfrequency conversion efficiency at high frequencies.

[0029] Generation of fourth and higher harmonics by non-linear optics:The MLQSW laser can also be used for generating fourth, fifth or higherharmonics, with the result that light having a high intensity andwavelengths in the ultraviolet or deep ultraviolet range is produced.The fourth harmonic of an MLQSW Nd:vanadate laser produces light of awavelength of 266 nm and the fifth harmonic light of 213 nm if suitablenon-linear optical materials, such as, for example, the borates BaB₂O₄(BBO), LiB₃O₅ (LBO) or CsLiB₆O₁₀ (CLBO) are used for frequencyconversion.

[0030] MLQSW optical-parametric generation: The non-linear opticalprocess of optical-parametric generation (OPG), of optical-parametricoscillation (OPO) or of optical-parametric amplification (OPA) can beused in combination with an MLQSW laser for achieving frequency shifts.For example, these processes can be used for obtaining visible light,for example the colors red, green and blue, which can be used for laserdisplays or applications in entertainment electronics. Similarly,“colors” which can be used for applications in molecular sensors or gassensors can be produced in the infrared range. The achievablecompactness of the MLQSW laser is in turn the basis for high compactnessof the overall system. The subsequent optical-parametric oscillation hasan optical length which is identical to the MLQSW laser length and istherefore as short as the length of the MLQSW laser resonator, resultingin a compact system.

[0031] Two-photon or multiphoton absorption effects: The MLQSW laser canalso be used in combination with two-photon or multiphoton absorptioneffects in a multiplicity of materials. This may be, for example,multiphoton stereolithography, in which the curing of the syntheticresin is effected by two-photon or multiphoton absorption. Similarly,optical two-photon storage media can be written on by means of MLQSWlasers. Here, the high repetition rate of the Q-switch envelope of themode-locked pulses leads to high achievable write rates compared withQ-switch lasers. In addition, the higher peak power results in in [sic]more efficient and faster writing and/or a lower current requirement. Inparticular, the MLQSW laser can be readily used in combination with anynon-linear optical process or any non-linear optical application whichhas a power index proportional to$\left( \frac{E_{p}^{3}}{t_{p}^{2}} \right) \cdot f_{rep}$

[0032] or to${\left( \frac{E_{p}^{n}}{t_{p}^{n - 1}} \right) \cdot f_{rep}},$

[0033] where E_(p) denotes the energy per pulse, t_(p) the pulseduration, f_(rep) the repetition rate of the pulses and where n>2. Owingto the high pulse energy and the short pulse duration of MLQSW lasers,the power index achieved is substantially higher than that achievablewith comparable continuous-wave mode-locked or Q-switch lasers. Inaddition, the MLQSW laser can be kept more compact and simpler than theother lasers.

[0034]FIG. 2 schematically shows an alternative embodiment which usesthe following components:

[0035] Pump means: The embodiment described uses a laser diode array 1a, which is focused by means of a transfer optical system 2 and adichroic mirror 3 into the laser medium 5 and which achieves a pumpintensity of about ˜20 kW/cm². This can be achieved in a compact manner,for example, by using a pump design as described in PCT Application No.PCT/EP00/05336 of Jun. 9, 2000, U.S. Application No. 60/146,472 of Jul.30, 1999 and U.S. application Ser. No. 09/489,964 of Jan. 24, 2000,which is hereby considered to have been disclosed in the context of thepresent invention. The design described there is compact and uses asmall number of optical elements focusing the pump light which isemitted by the laser diode array 1 a. Alternative realizations usefiber-optic means for inputting the pump light of the laser diodes intothe laser medium 5. The design shown in FIG. 2 produces a mode-lockedQ-switch laser emission S comprising laser pulses of approx. 5-10 psduration and having a repetition rate in the vicinity of 1 MHz. Thishigh repetition rate is sufficiently high for many applications andfulfils the needs for the high repetition rates as required for manylaser applications and for which the repetition rates of a few tens ofkHz of the typical Q-switch laser are not sufficient.

[0036] Laser medium: The laser medium 5 used is Nd:vanadate. This mediumhas a relatively high gain cross-section and also a relatively highsmall-signal gain which is proportional to the product of life of theupper laser level and effective cross-section of the emission. Thesmall-signal gain determines how fast the pulse build-up takes placewithin the envelopes generated by the Q-switch. If Nd:vanadate is used,pulse duration of the order of magnitude of a few picoseconds can beachieved owing to the emission bandwidth. However, depending on therespective laser application, other laser media may also be suitable.The neodymium doping of the Nd:vanadate is chosen sufficiently low (e.g.Nd doping <1% for absorbed pump powers of >5 W) so that the absorptionlength is long enough to avoid destruction of the crystal at full pumppower. On the other hand, the doping is chosen sufficiently high (e.g.Nd doping >0.1%) so that a sufficient proportion of the pump light isabsorbed in the laser medium. Alternative laser media are Nd:YLF,Nd:YAG, Nd:glasses, Cr:LiSAF, Cr:LiCAF, Cr:LiSGAF, Yb:YAG, Yb:glasses,Nd:vanadate operated for the 917 nm line, Nd:vanadate operated for the1340 nm line, Nd:YAG operated for the 946 nm line, or any other lasermaterial which has sufficient absorption in a range for which laserdiodes are available. Some of these materials have the advantage thatthey have a laser emission at wavelengths which are substantiallyshorter than the 1046 nm line of Nd:YAG or of Nd:vanadate, which isparticularly useful for some applications.

[0037] Laser resonator: A laser cavity having a set of highly reflectivelaser mirrors 7 a-7 c is formed, which mirrors are positioned around thelaser medium 5 in such a way that the total length of the cavity isabout 20 cm, which corresponds to a resonator revolution time of 1.33 nsor a pulse repetition frequency of about 750 MHz. That side of the lasermedium 5 which is on the outside relative to the resonator is in theform of a mirror having a reflectivity of 98%, based on the resonator(for 1064 nm) and a transmittance of >90% for the pump light. Thedistance between this surface of the laser medium 5 and the mirror 7 ais 41 mm, the respective distance between the mirrors 7 a and 7 b or 7 band 7 c is 61 mm, and the distance between mirror 7 c and SESAM 7 d isonce again 41 nm. The mirrors 7 a to 7 c are highly reflective and havethe following radii of curvature (ROC):

[0038]7 a 65 mm with respect to y axis, ∞ with respect to x axis,

[0039]7 b ∞ with respect to both axes and

[0040]7 c 100 mm with respect to both axes.

[0041] The stated values for the mirrors are to be understood as beingspecific for the setup shown by way of example and the components used,in particular the laser medium used. Other implementations require otherdesigns.

[0042] A guideline for the implementation is to adjust the laser mediumand light spot of the pump beam as exactly as possible in order toachieve a maximum amplification. This can be achieved, for example, byan arrangement as described in D. Kopf, K. J. Weingarten, G. Zhang, M.Moser, A. Prasad, M. A. Emanuel, R. J. Beach, J. A. Skidmore, U. Keller,Invited Paper, “High-average-power diode-pumped femtosecond Cr:LiSAFlasers”, Applied Physics B, vol. 65, pages 235-243, 1997, which-ishereby considered to have been disclosed in the context of the presentinvention. This laser resonator is extremely compact in comparison withmany typical mode-locked laser resonators, which have, for example, arepetition rate of about 80 MHz. If the 20 cm long resonator of thisarrangement shown in FIG. 2 is operated in MLQSW mode instead of withthe conventional continuous-wave mode-locking, it is still possible,with appropriately high compactness, to achieve peak powers which are ofthe same order of magnitude as in the case of a continuous-wavemode-locked laser resonator which is about ten times longer. Q-switchmode-locking can just as easily be used as an alternative in a longlaser resonator, with the result that higher peak powers are achievedthan in the case of corresponding operation with continuous-wavemode-locking.

[0043] Saturable absorber: The cavity contains a saturable absorber. Fora laser wavelength of 1064 nm, this may be, as represented here by theSESAM 7 d, a saturable semiconductor absorber which uses, as a saturablematerial, indium gallium arsenide which is embedded in a structure ofgallium arsenide and aluminum arsenide and which in its totality acts asa saturable absorber mirror (cf. “Semiconductor Saturable AbsorberMirrors (SESAMs) for Femtosecond to Nanosecond Pulse Generation inSolid-State Lasers”, Ursula Keller, et al., IEEE Journal of SelectedTopics in Quantum Electronics, Vol. 2, No. 3, September 1996).optionally, dielectric coatings can be applied to the surface of thestructure, with the result that the saturation parameters and themodulation depth can be influenced. The saturable absorption effect,expressed in units of the modulation depth, can range from fractions ofa percent to a few percent of the incident laser light. The modulationdepth can be used as a design parameter for designing the final laserparameters, such as, for example, the frequency of the Q-switch pulses,the pulse duration, etc. In the preferred embodiment of FIG. 2, amodulation depth of about 0.5% and a saturation flux of about 100microjoules/cm² (±50%) are used, which, in the Nd:vanadate embodiment,result in a repetition rate of the Q-switch envelopes of the mode-lockedpulses of about 1 MHz.

[0044] Alternative optical switches: According to the prior art,saturable absorbers have the advantage of particularly short switchingtimes. In principle, however, other optical switches, such as, forexample, mechanical switches, electrooptical switches or acoustoopticalswitches, can be used for producing a Q-switch. However, the switchingtimes of the mechanical systems are in the microsecond range, so thatuse for a laser according to the invention currently does not appearpossible. On the other hand, the electrooptical switches fully approachthe switching times achievable with saturable absorbers, for examplewith the use of the Pockel or Kerr effect. If a saturable absorber isdispensed with, the mode-locking can also be achieved by active lockingor by synchronous pumping. In the case of active locking, a lossmodulation is produced by an externally controlled modulator in thevicinity of a resonator mirror. Suitable components are available in theform of electrooptical and acoustooptical modulators. The synchronouspumping is effected, so to speak as a counterpart of the modulation ofthe losses, by a periodic modulation of the amplification, which can beeffected, for example, by synchronous pumping of the laser with themode-locked pulse series of another laser.

[0045] Mode-locked Q-switch (MLQSW) operation of the laser: A saturableabsorber having a short relaxation time of the order of magnitude equalto or less than the resonator revolution time, as possessed by mostInGaAs/GaAs-based saturable semiconductor absorbers (SESAM), often leadsto a mode which is referred to as mode-locked Q-switch. This is acombination of pure mode-locking, in which a stable pulse series havinga pulse repetition rate corresponding to the resonator revolution timeleaves the laser, and a pure Q-switch, in which giant pulses which aresubstantially longer than the resonator revolution time form. Themode-locked Q-switch pulse series is a mode-locked pulse series whoseenvelope is greatly modulated in the form of a Q-switch pulse. Thisusually undesired effect has the advantage that peak powers which aremuch higher than those which are achieved by comparable laser resonatorswith pure Q-switch or pure mode-locking are achieved. Since themode-locked pulse series is “chopped” and the emission in giant pulses(the term “giant pulse” is used on the basis of the designation of“giant-pulse lasers” for Q-switch lasers, cf. for example F. K.Kneubuhl, M. W. Sigrist, “Laser”, [Lasers], page 203, TeubnerStudienbücher, Stuttgart, 1991), the peak powers in the MLQSW mode arevery much higher, i.e. often up to an order of magnitude, than even thepowers achievable in the case of operation with continuous-wavemode-locking. MLQSW can therefore be used for obtaining extremely highpeak powers in comparison with mode-locking or the Q-switch for acomparable laser (such as, for example, with the same cavity length).These high peak powers are then suitable in particular for efficientfrequency conversion, such as, for example, UV generation. Inparticular, the generation of harmonics of the third or of a higherorder is supported by the high power.

[0046] MLQSW laser resonator having high repetition rates of theQ-switch envelope of the mode-locked pulses: MLQSW results in a typicalQ-switch pulse frequency which is of the order of magnitude of from afew tens of kHz to the range of 100-200 kHz. Many applications requirehigher rates. In order to obtain a high pulse repetition rate of theMLQSW laser resonator, the laser cavity can be designed in such a waythat the losses during resonator revolutions are low, for example of theorder of magnitude of a few percent or even less. In addition, shortcavities can substantially increase the repetition rate of the Q-switchenvelope of the mode-locked pulses.

[0047] Particularly compact MLQSW laser resonators: Many applicationsrequire or prefer particularly compact laser designs in which compactand hence short laser resonators are preferred. If such a short laserresonator is exclusively mode-locked, the achievable peak power is,however, lower than in a longer resonator. MLQSW operation can thereforesolve the problem of insufficient peak power from short laser resonatorsfor subsequent non-linear applications.

[0048]FIG. 3 shows the pulse series of an Nd:YLF MLQSW laser accordingto the invention, having a design described in FIG. 2. At a pump powerof 16 W, a laser output power of 4.02 W is achieved, the Q-switchrepetition rate being 350 kHz and the repetition rates of themode-locking being about 700 MHz. The saturable semiconductor absorber(SESAM) used has a modulation depth of about 0.15% and a saturation fluxof about 1 mJ/cm². The curve was recorded by measuring the photodiodesignal, which measures the laser beam and the pulse series, using aSpectrum Analyzer HP E4401 B from Hewlett-Packard as a measuringapparatus.

[0049] In the diagram, the frequency spectrum of the photodiode signalis plotted against the frequency in order to illustrate the noise-freeMLQSW operation. The mean frequency component of about 705 MHzillustrates the mode-locking repetition frequency corresponding to theresonator revolution frequency. While in pure mode-locking no side bandsoccurs, strong side bands in this case indicate the Q-switch and theassociated strong modulation of the mode-locked pulse series. From thefrequency interval of the side bands, the Q-switch repetition rate of350 kHz can be read. The fact that the frequency components are clearlyfully modulated corresponds to noise-free MLQSW operation.

[0050]FIG. 4 schematically shows the use of light which is generated bymeans of a frequency conversion light source according to the invention,according to the basic principle of FIG. 1.

[0051] A frequency conversion light source 10 comprising an MLQSW lasersource with subsequent frequency conversion to the ultraviolet rangeproduces laser emission S which is inserted into a scan apparatus 13 viaa deflection mirror 11 and an optical switch 12, which can be realized,for example, electrooptically or electroacoustically. By means of thescan apparatus, structures are drawn in an absorbing medium, e.g. aUV-sensitive synthetic resin. In interaction with the UV light, theexposed zones change their physical or chemical properties, for examplethrough curing.

[0052] Of course, the figures shown represent one of the manyembodiments and a person skilled in the art can derive alternative formsfor implementing the laser design, for example with the use of otherlaser media or resonator components. In particular, it is possible todesign the pumping of the laser by other methods over and above theexamples given or to arrange the necessary components in another manner.

1. A laser for emitting laser pulses having high pulse powers and high pulse rates, comprising an amplifying laser medium, a laser resonator having at least one resonator mirror and at least one output coupler, a pump source for pumping the laser medium and means for producing mode-locking, wherein means for producing a Q-switch, in particular having a pulse rate of above 100 kHz, are present and the pump source has a power of at least 5 W for pumping the laser medium.
 2. The laser as claimed in claim 1, wherein the means for producing mode-locking are designed in such a way that the pulse rate of the mode-locking is of the order of magnitude of from 2.5 MHz to several GHz.
 3. The laser as claimed in any of the preceding claims, wherein the length of the laser resonator is less than 0.75 m.
 4. The laser as claimed in any of the preceding claims, wherein the means for producing a Q-switch have at least one electrooptical or acoustooptical switch.
 5. The laser as claimed in any of the preceding claims, wherein the means for producing the mode-locking have at least one active, externally controlled modulator or, in combination with the pump source, are designed for synchronous pumping.
 6. The laser as claimed in any of the preceding claims, wherein the means for producing a Q-switch and/or the means for producing the mode-locking have at least one saturable absorber.
 7. The laser as claimed in claim 6, wherein the saturable absorber consists of semiconductor material which comprises at least one of the following materials indium gallium arsenide gallium arsenide aluminum arsenide indium gallium arsenide phosphide.
 8. The laser as claimed in claim 6 or 7, wherein the saturable absorber has dielectric coatings on at least one of its surfaces.
 9. The laser as claimed in any of claims 6 to 8, wherein the saturable absorber has a relaxation time less than or equal to the resonator revolution time.
 10. The laser as claimed in any of claims 6 to 9, wherein the saturable absorber has a modulation depth of from 0.1% to 10% and a saturation energy flux of 20-2000 microjoules/cm², preferably of 100 microjoules/cm².
 11. The laser as claimed in any of claims 6 to 10, wherein at least one resonator mirror is in the form of a saturable absorber.
 12. The laser as claimed in any of the preceding claims, wherein the means for producing a Q-switch and the means for producing mode-locking are combined in one component.
 13. The laser as claimed in any of the preceding claims, wherein the laser medium comprises one of the following materials Nd:vanadate, Nd:YLF, Nd:YAG, Nd:glass, Cr:LiSAF, Cr:LICAF, Cr:LiSGAF, Yb:YAG, Yd:KGW, Yb:KYW or Yb:glass.
 14. The laser as claimed in any of the preceding claims, wherein the pump source comprises at least one laser diode or a laser diode array.
 15. A method for producing laser pulses having high pulse powers and pulse rates above 100 kHz, comprising a laser system consisting of an amplifying laser medium, a laser resonator having at least one resonator mirror and at least one output coupler, a pump source for pumping the laser medium and means for producing mode-locking, in which the laser system produces laser pulses, a laser emission by the laser system being influenced by the means for producing mode-locking in such a way that mode-locked laser pulses are produced, wherein means for producing a Q-switch are present and the mode-locked laser pulses are amplitude-modulated according to the pulse rate of the Q-switch by the means for producing a Q-switch.
 16. The method as claimed in claim 15, wherein the laser emission is influenced by the laser system in such a way that mode-locked laser pulses having a pulse rate of the order of magnitude of from several 2.5 MHz to several GHz are produced.
 17. The method as claimed in claim 15 or 16, wherein the mode-locked laser pulses having a Q-switch pulse rate of the order of magnitude of from 10 kHz to 1 MHz are amplitude-modulated.
 18. The use of a laser as claimed in any of claims 1 to 14 as a laser source for utilizing non-linear optical effects, in particular for frequency multiplication, for two-photon or multiphoton absorption effects or for optical parametric generation, oscillation or amplification.
 19. A frequency conversion light source comprising a laser for emitting pulsed laser light having high pulse powers and high pulse rates, comprising an amplifying laser medium, a laser resonator having at least one resonator mirror and at least one output coupler, a pump source for pumping the laser medium and means for producing mode-locking; means for frequency conversion of the laser light, wherein means for producing a Q-switch, in particular having a pulse rate above 100 kHz, are present.
 20. The frequency conversion light source as claimed in claim 19, wherein the pump source has a power of at least 5 W for pumping the laser medium.
 21. The frequency conversion light source as claimed in claim 20, wherein the means for producing mode-locking is designed so that the pulse rate of the mode-locking is of the order of magnitude of from 2.5 MHz to several GHz.
 22. The frequency conversion light source as claimed in either of claims 19 and 21, wherein the length of the laser resonator is less than 0.75 m.
 23. The frequency conversion light source as claimed in any of the preceding claims 1 to 22, wherein the means for producing a Q-switch have a least one electrooptical or acoustooptical switch.
 24. The frequency conversion light source as claimed in any of the preceding claims 19 to 23, wherein the means for producing the mode-locking have at least one active, externally controlled modulator or, in combination with the pump source, a design to give synchronous pumping.
 25. The frequency conversion light source as claimed in any of the preceding claims 19 to 24, wherein the means for producing a Q-switch and/or the means for producing the mode-locking have at least one saturable absorber.
 26. The frequency conversion light source as claimed in any of the preceding claims 19 to 25, wherein the saturable absorber consists of semiconductor material which comprises at least one of the following materials indium gallium arsenide gallium arsenide aluminum arsenide indium gallium arsenide phosphide.
 27. The frequency conversion light source as claimed in any of the preceding claims 19 to 26, wherein the saturable absorber has dielectric coatings on at least one of its surfaces.
 28. The frequency conversion light source as claimed in any of the preceding claims 19 to 27, wherein the saturable absorber has a relaxation time of less than or equal to the resonator revolution time.
 29. The frequency conversion light source as claimed in any of the preceding claims 19 to 28, wherein the saturable absorber has a modulation depth of from 0.1% to 2% and a saturation energy flux of about 100 microjoule/cm².
 30. The frequency conversion light source as claimed in any of the preceding claims 19 to 29, wherein at least one resonator mirror is in the form of a saturable absorber.
 31. The frequency conversion light source as claimed in any of the preceding claims 19 to 30, wherein the means for producing a Q-switch and the means for producing mode-locking are combined in one component.
 32. The frequency conversion light source as claimed in any of the preceding claims 19 to 31, wherein the laser medium comprises one of the following materials Nd:vanadate, Nd:YLF, Nd:YAG, ND:glass, Cr:LiSAF, Cr:LiCAF, Cr:LiSGAF, Yb:YAG, Yb:KGW, Yb:KYW or Yb:glass.
 33. The frequency conversion light source as claimed in any of the preceding claims 19 to 32, wherein the pump source comprises at least one laser diode or a laser diode array.
 34. The frequency conversion light source as claimed in any of the preceding claims 19 to 33, wherein the means for frequency conversion have elements with non-linear optical effects, in particular comprising KnbO₃ [sic], BaB₂O₄ (BBO), LiB₃O₅ (LBO) or CsLiB₆O₁₀ (CLBO). 